One of the cheapest prime lenses you can get for the Canon EF type mount is the 50mm F1.8 model 2 ore the original 50mm F1.8. The lens is very sharp, but still cheap at a price range everybody can be part of. The lens will take your image quality and photographing to the next level, and should be the first one to get when upgrading from the kit-lens ore accompanying your camerakit.
I was so lucky to pick up a original version of the 50mm F1.8 in near mint condition with original packing and manual for only 46 USD (300 DKK).
My main camera is the Canon 5D original /march 1 and when i saw the original 50mm for sale I just had to have it. This original version fits very well the clunky and slow version of my 5D. Now both the camera and the lens are clunky, slow and loud.
I already own the second version of this lens and bought it from new some years ago when I had my Canon 400D (Rebel XTI). The second version produces images in the same quality.
Both the original and the new version of the 50mm F1.8 are mostly made out of plastic and in the lower end of build quality. They both are very noisy when auto focusing and some people will call the autofocus speed slowmotion.
The original version has though a better build quality than the second version and was produced only from 1987-1990. The mount is made out of metal and not plastic, and the overall build feeling is slightly better. But outofocus speed and sound are the worst. Autofocus is slow by todays standards. You need to think about what you are taking pictures of and compose your shoot before you take it. This has somewhat a analog feeling to it, and is not suitable for sports photography ore any high speed photography. Don't mention the sound, this one is loud, very loud when autofocusing, and definitely not for silent and stealth picture taking.
But all of this doesn't matter, because the picture quality for both lenses is very good and can hardly be match by any other lens on the market for the same price.
The images where all taken with the Canon 5D and the original 50mm F1.8. The images are edited with the OpdenSource editor Darktable for Ubuntu (a review and walkthrough maybe coming in the future). All images are only edited very slightly with adjustment of contrast/highlights, cropping, a little vignetting, and some level adjustments.